When incident radiation interacts with matter, it may undergo a process called scattering. As described in J. B. Ingle, Jr. and S. R. Crouch, "Molecular Scattering Methods," Chapter 16 in Spectrochemical Analysis, 1988, Prentice-Hall, Englewood Cliff N.J., pp 494-499, scattering may be elastic, i.e., the wavelength of the scattered radiation is unchanged from that of the incident radiation, or inelastic, i.e., the scattered radiation has a wavelength different from that of the incident radiation.
In one form of elastic radiation scattering, referred to as Rayleigh scatter, the dimensions of the scattering particles, i.e., atoms and molecules, are much smaller than the incident beam wavelength. In general, Rayleigh scatter is inversely proportional to the fourth power of the wavelength of the incident light. Liquids exhibit significantly stronger Rayleigh scatter than do gasses.
One type of inelastic radiation scattering is referred to as Raman scatter; incident photons are scattered with either a gain or loss of energy, and the energy difference between the scattered and incident radiation is commonly referred to as the Raman shift. The Raman shift spectrum represents the energy of various molecular vibrations and conveys chemical and molecular information regarding the matter studied. Raman spectrometry is widely used in the analysis of various materials and is capable of providing both qualitative and quantitative information about the composition and/or molecular structure of chemical substances.
Raman scattering signals are very weak, much weaker than Rayleigh scattering signals. Typically a few Raman scattering photons exist among millions of elastically scattered photons. This small Raman signal among the large elastically scattered signals places severe demands on the instrumentational design of any spectrometer used to collect Raman spectra.
A Raman spectrometry apparatus typically comprises a laser excitation source of monochromatic light, a probe, and a fiber optic cable that includes transmission and reception fiber channels connecting the laser with the probe. The probe may be remotely located from the laser light source; it may, for example, be situated within a chamber such as a reactor or a pipe where a chemical reaction involving solids, liquids, gasses, or mixtures thereof is occurring. The fiber optic cable includes transmission and reception fibers. The output of the laser is conveyed by a transmission fiber channel to the probe, exits the probe, and illuminates the material within the reaction chamber. Raman scattering resulting from irradiation of the material is conveyed by a reception fiber channel to a detector and spectrograph included in the spectrometry apparatus.
Lasers are classified according to their power output, from very low power "exempt" lasers of Class I to high power lasers of Classes III and IV, whose output range from about 1 mW to greater than 500 mW. Lasers used as excitation sources for Raman spectrometry are frequently Class III or IV and therefore have output energy levels that present a potential hazard. If the probe were inadvertently removed from a material being measured, the high laser output could damage the cornea or retina of an operator's eyes and could also ignite flammable substances in the vicinity, causing a fire or explosion. To ensure personnel safety and minimize the hazards of fire or explosion, it would be highly desirable to have a reliable control for automatically turning off the laser if the probe were to be removed from the material under examination or if a break were to occur in a fiber optic channel. Such a control is provided by the present invention.